The object of the present invention is a lithium-rich carbonaceous substance comprising lithium, sodium, carbon and oxygen, which can be used in particular as a negative electrode in a lithium electrical accumulator.
It applies to the production of accumulators with a high mass energy which are of great interest for the development of portable electronics and in the long term for the manufacture of electric vehicles. Currently, for these applications, the choice seems to relate to lithium ion batteries because of their very high potential energy per unit mass. For the anode part of these batteries, numerous studies have shown that it is possible to use carbonaceous compounds and in particular graphite.
This is because graphite with a lamellar structure has the property, under the effect of an electric current in an appropriate electrolytic medium, of inserting lithium ions in its structure. The substance obtained is a graphite insertion compound, that is to say there is an invasion of the vacant spaces between the graphite lamellae, also referred to as van der Waals spaces, by the lithium ions.
At the present time, the insertion compound of lithium in graphite which is the richest known has one lithium atom for six carbon atoms, which corresponds to the total formula LiC6 and has an electrochemical capacitance of 372 mA.h.gxe2x88x921. This compound LiC6 is described in Carbon, volume 13, pages 337-345, 1975 [1].
Through FR-A-2 697 261 [2], a polyacetylene-lithium insertion compound is known in which the C/Li ratio is 6, as in the case of the insertion compound of lithium in graphite LiC6.
In order to improve the performance of lithium batteries, it would however be highly advantageous to use, as a negative electrode, compounds containing even more lithium than these known compounds, in order to obtain a higher electrochemical capacitance.
The object of the present invention is precisely a novel carbonaceous substance containing more lithium than any known compounds, which can be used as a negative electrode in a lithium ion battery.
According to the invention, this lithium-rich carbonaceous substance has the following formula:
LiNaxCyOzxe2x80x83xe2x80x83(I)
in which x, y and z are such that:
0.4xe2x89xa6xxe2x89xa60.6
2.5xe2x89xa6yxe2x89xa63.5
0.2xe2x89xa6zxe2x89xa61
The carbonaceous substance of the invention thus contains more lithium than the insertion compound of lithium in graphite LiC6 since the C/Li ratio is situated within the range 2.5 to 3.5, which corresponds to a significant increase in lithium content and an increased electrochemical capacitance (xcx9c744 mA.h per gram of carbon).
This lithium-rich carbonaceous substance can be obtained easily from a graphite-sodium-oxygen compound such as the one described by M. El Gadi et al in J. Mol. Cryst. Liquid. Cryst., 244-245, 1994, page 41[3].
The formula of this compound deduced from elemental analyses is:
C4.75xc2x10.05NaO0.35xc2x10.05
According to the invention, the lithium-rich carbonaceous compound is prepared by inserting lithium into a graphite-sodium-oxygen compound of this type. This insertion can be carried out either chemically or electrochemically.
The insertion by chemical method can be effected by means of a method similar to the one described by A. Essaddek et al in C. R. Acad. Sci., Paris, 1. 319, Series II, 1994, pages 1009-1012 [4]), starting from the compound NAO0.5C6 with an ideal theoretical formula derived from crystallographic analysis. In this case, the graphite-sodium-oxygen compound is put in contact with liquid lithium at a temperature and for a period sufficient to obtain the lithium-rich carbonaceous substance of the invention.
It is also possible to start from a graphite-sodium-oxygen compound prepared by putting in contact, in a sealed chamber containing no oxygen, graphite with sodium containing a small quantity of oxygen, at a temperature of 460 to 480xc2x0 C., and preferably 470xc2x0 C.
The graphite used for this preparation can be natural or artificial graphite, in flake or powder form with variable granulometries.
By way of examples of graphites which can be used, pyrographite of the PGCCL type supplied by Carbone Lorraine and highly orientated pyrographite HOPG supplied by Union Carbide can be cited.
The small quantity of oxygen present in the sodium can represent 0.5 to 2% by weight for a volume of sodium of 1 to 2 cm3.
Preferably, according to the invention, the insertion of the lithium in the graphite-sodium-oxygen compound is effected electrochemically. In this case, an electrolytic cell with two electrodes is used, one of which is made of lithium and the other one of which consists of the graphite-sodium-oxygen compound, the two electrodes being immersed in an electrolyte comprising a lithium salt.
The electrolyte generally comprises a non-aqueous solvent, for example ethylene carbonate. The lithium salts which can be used may be of different types, but lithium perchlorate is preferred.
In order to effect the electrochemical insertion, the operation is preferably carried out at a temperature of 20 to 90xc2x0 C. and a potential of 0 volts is imposed between the two electrodes, which can be achieved by connecting the two electrodes by simple short-circuit.
The lithium-rich substance of the invention can be used in particular as a negative electrode in a lithium electrical accumulator.
Thus another object of the invention is an accumulator comprising a negative electrode based on lithium, a positive electrode and an electrolyte conductive by lithium ions in which the negative electrode comprises the lithium-rich carbonaceous substance of the invention.
In this accumulator, the positive electrode can be produced from various materials such as oxides, sulphides or oxysulphides.
By way of example of oxides which can be used, it is possible to cite vanadium oxide V2O5, nickel oxide NiO2, cobalt oxide CoO2, mixed oxides of cobalt and nickel, manganese oxides, molybdenum oxide MoO3, chromium oxides and vanadium bronzes MxV2O5 with M representing iron, sodium, potassium, lithium, silver, aluminium, chromium, barium, nickel or cobalt.
By way of examples of sulphides which can be used, titanium sulphide TiS2, molybdenum sulphide MoS2 and mixed sulphides of nickel and molybdenum can be cited.
By way of examples of oxysulphides which can be used, molybdenum and titanium oxysulphides can be cited.
In this electrical accumulator, the electrolyte used generally consists of a solution of lithium salt in a suitable organic solvent.
The organic solvents which can be used are for example propylene carbonate, ethylene carbonate, dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxymethane, dimethoxyethane, N,N-dimethylformamide, sulfolane and mixtures thereof.
The lithium salts which can be used are for example lithium perchlorate LiClO4, lithium hexafluorophosphate LiPF6, lithium hexafluoroarseniate LiAsF6, lithium trifluoromethanesulfonate LiCF3SO3 and lithium tetrafluoroborate LiBF4.
In the electrolytic accumulator, it is also possible to use as an electrolyte, instead of a solution of lithium salt in an organic solvent, solid electrolytes or polymers conductive by lithium ions.
By way of examples of solid electrolytes, lithium glasses can be cited, obtained for example from P2S5, Li2S and LiI or B2S3, Li2S and LiI.
The polymers conductive by lithium ions can consist for example of poly(ethylene oxide) or poly(propylene oxide) containing a lithium salt such as the salts described above.
In an accumulator of this type using a liquid electrolyte, generally a separator is disposed between the electrodes, and this can consist of a microporous film produced for example from polypropylene or polyethylene.
This accumulator can be produced in the form of a cylindrical accumulator having a spiral winding of the two electrodes separated possibly by the separator. It can also be produced in the form of an accumulator of the prismatic type with plane electrodes facing each other and possibly a separator disposed between these electrodes.
Other characteristics and advantages of the invention will emerge more clearly from a reading of the following description, given of course for illustrative purposes and non-limitatively, with reference to the accompanying drawings.